Freshman Sen. Jeff Merkley (D-Ore.) is returning the favor for Sen. Russ Feingold (D), who is up for re-election to a fourth term in 2010. In an e-mail to his supporters, Merkley credits Feingold with being “squarely in my corner— during his 2008 victory over a Republican incumbent and asks his supporters to donate to the Wisconsin Democrat’s re-election campaign.

“It is because of all that he did to help me, and all that he stands for in the US Senate that today I am asking you to join me in standing in Senator Feingold’s corner,— Merkley wrote.

But unlike Merkley’s competitive race, Feingold has yet to attract an opponent for 2010. Feingold ended last year with just over $2.5 million in his campaign’s bank account.